Method and apparatus for synchronizing a radio telemetry system by way of transmitted-reference , delay-hopped ultra-wideband pilot signal

ABSTRACT

A time-division-multiplexed radio communication system and method uses transmitted-reference, delay-hopped (TR/DH) ultra-wideband (UWB) broadcast signal to provide a pilot signal to all mobile devices in a coverage area from which time synchronization is derived. Using this TR/DH UWB pulse pilot signal and low-complexity demodulation in the mobile devices, the mobile devices utilize a simple signal detection algorithm to acquire synchronization with the pilot signal. As a result, all devices in a local area network become synchronized to the system&#39;s bit clock. This greatly reduces the search space required for signal acquisition, receiver signal processing complexity, and length of message preambles required to synchronize the base station receiver to a transmission from any of the mobile devices.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The subject matter of this application is related to copendingU.S. patent application Ser. No. 09/753,443 filed Jan. 3, 2001, by H. W.Tomlinson, Jr., J. E. Hershey, R. T. Hoctor, and K. B. Welles, II, for“Ultra-Wideband Communication System” (GE Docket RD-27,754), copendingU.S. patent application Ser. No. 09/974,032 filed Oct. 10, 2001, by R.T. Hoctor, D. M. Davenport, A. M. Dentinger, N. A. Van Stralen, H. W.Tomlinson, Jr., K. B. Welles, II, and J. E. Hershey for “Ultra-WidebandCommunication System and Method Using a Delay-Hopped, Continuous NoiseTransmitted Reference” (GE Docket RD-28,759), copending U.S. patentapplication Ser. No. 09/973,140 filed Oct. 9, 2001, by R. T. Hoctor, J.E. Hershey and H. W. Tomlinson, Jr., for “Transmitter Location forUltra-Wideband, Transmitted-Reference, CDMA Communication System” (GEDocket RD-27,855), and copending U.S. patent application Ser. No. ______filed , ______, 2002, by R. T. Hoctor and S. Hladik for “Synchronizationof Ultra-wideband Communications Using a Transmitted-reference Preamble”(GE Docket RD-28,133) all of which are assigned to the assignee of thisapplication. The disclosures of U.S. patent application Ser. Nos.09/753,443, 09/974,032, 09/973,140, and Ser. No. ______ (GE DocketRD-28,133) are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a transmitted-reference, ultra-wideband(TR-UWB) radio communication system and, in particular, to a scheme forusing such a signal as a pilot signal that all radio transmitters in acoverage area can receive and derive time synchronization therefrom.

[0003] The use of time division multiple access (TDMA) to synchronizemultiple transmitters that wish to make use of the same channel iswell-known in the communications art. In the TDMA scheme, multipletransmitters make use of a communication channel serially, each onereceiving a time slot during which it has exclusive use of the channel.Another scheme for the coordination of multiple transmitters that isrelated to TDMA is slotted random access. An example of this type oftechnique is the slotted ALOHA media access approach. In a slottedrandom access system, transmitters may use the channel at any time theylike, but their transmissions must start at a time determined by acertain clock. This clock determines the slots at which transmissionsmay take place.

[0004] One problem associated with both the TDMA scheme and the slottedrandom access scheme is the establishment and maintenance of a commontime base to regulate the transmission times of the transmitters. Ingeneral, some kind of clock has to be distributed to all thetransmitters to allow them to transmit at the right time. In someradio-based TDMA systems, this is done by assigning a time slot to themaster receiver, during which it broadcasts a message, and having all ofthe transmitters receive this signal and use its time of arrival at thetransmitter as a time mark. Thus, some portion of the channel bandwidthis used to transmit timing information rather than data. Thisinformation-bearing bandwidth need not be lost if the timing informationcan be distributed to the transmitters using some kind of out-of-bandchannel.

[0005] Ultra-wideband (UWB) communications is the name given to a typeof radio transmission which works by transmitting pulses; in fact,another name for this type of communications is “impulse radio”. (See M.Z. Win and R. A. Sholtz, “Impulse radio: how it works”, IEEE Comm.Letters, vol. 2, pp. 36-38, February 1988, and L. W. Fullerton, “Spreadspectrum radio transmission system”, in U.S. Pat. No. 4,641,317.)

[0006] Recently, a new UWB communications scheme, calledtransmitted-reference, delay-hopped (TR/DH) ultra-wideband, has beeninvented, as described in copending U.S. patent application Ser. Nos.09/753,443 (GE Docket RD-27,754) and 09/974,032 (GE Docket RD-28,759).The term “transmitted reference” refers to the transmission andreception of multiple pulses in such a manner that synchronization withthe individual pulses is unnecessary. Transmitted reference UWBtransmits pulses in pairs, and thereby induces a correlation at thereceiver that can be measured by standard means. The term “delay-hopped”refers to a code-division multiple access (CDMA) scheme which usestransmitted-reference UWB.

[0007] In addition to the standard ultra-wideband (or “impulse radio”)version of TR/DH, the inventors have invented and experimented with aversion of TR/DH that uses wideband noise as a carrier, rather thanimpulse trains. This version of the invention induces correlation at thereceiver by transmitting the sum of two versions of a wideband,continuous noise, separated by a log known to the receiver. Thisinvention has advantages in that the noise carrier may be easier togenerate than the impulse train carrier, and it is described incopending U.S. patent application Ser. No. 09/974,032 (GE DocketRD-28,759). In addition to transmitting information, TR/DH UWB can beused to establish a time mark with respect to a clock at the receiver;in this way it can be used to distribute a clock. This application isdescribed in copending U.S. patent application Ser. Nos. 09/973,140 (GEDocket RD-27,855) and ______ (GE Docket RD-28,133).

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention solves the signal acquisition problemassociated with time-division-multiplexed or time-slotted communicationsby utilizing a TR/DH ultra wideband transmission as a pilot signal thatall devices in the coverage area can receive and derive synchronizationtherefrom. The transmitted-reference ultra-wideband pulse pilot signalis broadcast from a central node or base station, located in thecoverage area, to all devices that are to have their transmission timessynchronized. The devices utilize a simple signal detection algorithm toacquire synchronization with the pilot signal to an accuracy of roughly10 nanoseconds. Then, signal acquisition may be completed by a fineacquisition algorithm, if required. As a result, all devices in a localarea network become synchronized to the system's bit clock. This allowseither a TDMA or slotted transmission to take place in the reverse, orinbound, direction, from the device to the central node.

[0009] This invention is of particular interest for hospital asset andpersonnel tracking, medical telemetry for ambulatory patient monitoring,and wireless local area network data communications for productivity andpatient-care quality enhancements.

[0010] It is also of interest for wireless process monitoring andcontrol applications.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a block diagram illustrating a one-cell systemarchitecture and signal paths on which the invention may be implemented;

[0012]FIG. 2 is a block diagram illustrating the air interface-outboundsignal structure;

[0013]FIG. 3 is a timing diagram showing the synchronization of inboundsignal to outbound signal's bit epoch;

[0014]FIGS. 4A and 4B are, respectively, flow charts of the process bywhich portable devices synchronize to an outbound signal from a basestation and the base station demodulator's operation;

[0015]FIG. 5 is a block diagram showing an example of an inbound burststructure;

[0016]FIG. 6 is a timing diagram illustrating the structure of anoutbound pilot signal;

[0017]FIG. 7 is an illustration of the timing relationships among theoutbound pilot signal, the inbound burst and the base station clock;

[0018]FIG. 8 is a block diagram of the mobile UWB receiver andnarrowband transmitter;

[0019]FIG. 9 is a block diagram of a pulse pair correlator for a lag D;

[0020]FIG. 10 is a block diagram of the first stage of the UWB TRdelay-hopped CDMA receiver, with illustrations of a set of outputs ofthe pulse pair correlators;

[0021]FIG. 11 is a block diagram of a simple example of the delay-hoppedcode correlator; and

[0022]FIG. 12 is a diagram illustrating the output of the DH codecorrelator.

DETAILED DESCRIPTION OF THE INVENTION

[0023]FIG. 1 illustrates a block diagram of one embodiment of theinvention. It comprises a base station 11, which transmits adelay-hopped, transmitted-reference ultra-wideband (TR/DH-UWB) pilotsignal (as described in copending U.S. patent applications Ser. Nos.09/753,443 (GE Docket RD-27,754) and 09/974,032 (GE Docket RD-28,759)),and one or more mobile radio devices 12 ₁, 12 ₂, 12 ₃, 12 _(n). Thesemobile radio devices include a receiver that can receive the TR/DH pilotsignal 5 and a transmitter for sending messages to the base station. Theburst signals 6 transmitted by the mobile radio devices are synchronizedwith periodic cell epochs that are carried by the pilot signal. As aresult, the uncertainties in the times of arrival of inbound bursts atthe base station may be greatly reduced by use of a slotted transmissionscheme. This in turn reduces the search interval required for signalacquisition, receiver signal processing complexity, and length ofmessage preambles. Alternatively, interference between inbound bursts inthe same frequency band can be eliminated by use of a TDMA scheme. In apreferred embodiment, the mobile devices' transmit signals withnarrowband modulation, such as Gaussian Minimum Shift Keying (GMSK).

[0024] The outbound signal transmitted by the base station 11 may be anunmodulated transmitted-reference, delay-hopped UWB, i.e., a pilotsignal that is only used by the mobile devices to obtain synchronizationwith the base station's symbol clock, or it may comprise both pilot andmessage (data) carrying components as shown in FIG. 2. The air interfacefor this invention is designed so that inbound bursts are coincidentwith a periodic epoch conveyed to the mobile devices by the base stationvia its outbound signal.

[0025] The TR/DH UWB modulation scheme provides multiple access capacitythrough the use of the delay-hopping CDMA codes. The fact that multipleDH codes can be transmitted simultaneously and received without errormeans that multiple uncoordinated TDMA or slotted systems can be insimultaneous operation using this scheme.

[0026] In one slotted random access scheme embodiment of the invention,the cell's epoch is any bit epoch of the received outbound signal. In apreferred embodiment of this invention illustrated in FIG. 3, the mobileradio transmits inbound messages beginning at a bit boundary of anyreceived bit in the outbound signal. The timing relationships among thetransmitted outbound signal, outbound signal received at a givenportable device, and the inbound message signals transmitted by severalportable devices are shown in FIG. 3 for an exemplary case. If M-arymodulation is used wherein M>2, inbound transmissions are synchronizedto any symbol epoch in the received outbound signal. Another alternativeembodiment is that the beginning of any specified fields in the receivedoutbound signal are used as a cell epochs. Yet another embodiment isthat the specified field be used to identify a particular mobile deviceor a group of mobile devices that are permitted to transmit. Thisfeature can be used to implement a TDMA system.

[0027]FIG. 4A is a flowchart that describes the method used by themobile devices to synchronize their transmissions with the cell'sbit-time epoch. When a mobile device has data to send at input block 41,its receiver synchronizes with the pilot signal broadcast by the basestation in function block 42. Then, function block 43, the mobile devicebegins its burst transmission at the estimated periodic bit epoch (orsymbol epoch) until the end of the message at output block 44.

[0028] Burst acquisition can be simplified in the base station by takingadvantage of the a priori knowledge that the beginning of inbound burstswill be found near the symbol epochs of the outbound pilot signal. FIG.4B is a flowchart of a base station demodulator's operation. When ademodulator is in the ready state 45 to acquire a new burst, it beginsthe acquisition process at the cell's periodic epoch in function block46. At the end of the acquisition interval, a decision is made indecision block 47 on whether or not a burst has been acquired. If not,the demodulator returns to the ready state 45. If a burst has beenacquired, the demodulator proceeds to demodulate the burst in functionblock 48. After the burst has been demodulated, the demodulator returnsto the ready state 45.

[0029] It should be emphasized that the flowcharts of FIGS. 4A and 4Bcan apply to either a TDMA system or a slotted random access systemimplemented using a TR/DH pilot signal. In the TDMA system, means mustbe provided in the outbound message structure to identify the mobiledevice or devices that may transmit. In the slotted system, no suchidentification is necessary, since the mobile devices themselves makethe decision of when to transmit. In the TDMA system, the pilot signalmust be structured so as to allow the device that “owns” the time slotto complete its transmission before other potentially interferingdevices are allowed to transmit. In the slotted random access system,transmissions are allowed to overlap, even if this causes interference;therefore, the slots in such a system can be more closely spaced thanthe duration of a message from the mobile devices.

[0030] One possible embodiment of the inbound burst is shown in FIG. 5.It comprises a preamble to aid in burst acquisition, a burst header anddata.

[0031] Synchronization of inbound bursts with the cell epochs conveyedby the outbound signal greatly reduces the uncertainty in burst time ofarrival, particularly in local area networks wherein propagation delaysare very small relative to a bit interval. As a result burst acquisitiontime by base station demodulators is greatly reduced. The timeuncertainty in the reception of an unsynchronized burst transmissionrequires that the receiver first detect the presence of the RF burst,then synchronize to its symbol timing, and finally synchronize to itsphase relative to a local phase source. In contrast, the uncertainty ininbound burst epochs at the base station in our invention is limited toapproximately twice the maximum propagation delay, which is typicallymuch smaller than a symbol interval in a wireless local area network.For example, the propagation delay for every 3 meters in range betweentransmitter and receiver is only 10 nanoseconds. Even at a range of 60meters the propagation delay is quite small (200 nanoseconds) comparedto a symbol interval for a 10 kbps link (100 microseconds). For thisexample, the time uncertainty at the base station is reduced by a factorof 500. This allows the receiver to dispense with burst detection andsymbol timing synchronization, although phase synchronization must stillbe accomplished.

[0032]FIG. 6 illustrates the outbound pilot signal utilized in thisinvention when the impulse radio version of TR/DH signal is used for thepilot signal. As shown in FIG. 6, each chip comprises a number N_(h) oftime-hopping intervals over which the delay between theinformation-bearing pulse and reference pulse is constant. Also as shownin FIG. 6, the delay between the information-bearing pulse and referencepulse during the k^(th) delay-hopping interval (or dwell interval) isdenoted T_(dk).

[0033] The timing relationships between the various signals in a systemthat utilizes an outbound ultra-wideband pilot signal to establish aslotted random access scheme are illustrated in FIG. 7. In this figure,the tick marks on the time axis delineate the bit intervals of theoutbound (base-to-mobile) signal referenced to the transmitter. Notethat the outbound bit interval T_(bo) may be different from the inboundbit interval T_(bi). By way of example, the bottom portion of FIG. 7shows a magnification of the timing of the received inbound signal (froma mobile transmitter) at a base station. In this example, the first bitof the inbound signal arrives at the base station T_(offset) secondsfrom the leading edge of the n th outbound bit boundary. The time offsetbetween an outbound bit epoch and the initial bit epoch of the receivedinbound signal is approximately equal to twice the propagation delaybetween the mobile and base transceivers. As a result, the maximumuncertainty in the time of arrival for an inbound message is determinedprimarily by the maximum range between the base and mobile units. Whenthe base and mobile transmit at the same bit rate, T_(offset) is thetime offset between any of the bit epochs in the received inboundmessage and the nearest epoch in the outbound signal.

[0034]FIG. 8 is a block diagram of a TR/DH receiver and narrowbandtransmitter such as would be used by the mobile device. The receiver fora TR/DH code word comprises an antenna 81 and an RF amplifier 82followed by a bank of pulse-pair correlators 83 ₁ to 83 _(n) and a DHcode word correlator 85. The analog output of each pulse pair correlatoris digitized by a corresponding one of the analog-to-digital converters(ADCS) 84 ₁ to 84 _(n) before being input to the all-digital DH codecorrelator 85. A typical value of the sample rate at which thisdigitization takes place would be in the range of 1 to 20 MHz, and wouldprovide at least 2 samples in each chip interval.

[0035] A pulse pair correlator is depicted in FIG. 9. A pulse-paircorrelation consists of a delay 91, a signal multiplier 92 and afinite-time integrator 93. The signal is split into two paths, one ofwhich is delayed by delay 91. The two versions of the received signalare multiplied in multiplier 92, and the product is integrated over aspecified time, T_(c), by integrator 93. The integration time is equalto the chip time. The delay is such that the leading pulse or noisecarrier of the delayed circuit path is registered in time with thetrailing pulse or noise carrier of the un-delayed circuit path. Thisnon-zero-mean product is integrated over a chip interval to produce achip signal.

[0036] The chip signals at the outputs of the bank of pulse paircorrelators are characteristically peaked as shown in FIG. 10. Theantenna 101 provides inputs to correlators 102 ₁ to 102 _(N) _(c) ,which comprise a bank of pulse-pair correlators shown in FIG. 9. Thesesignals are of duration approximately equal to twice the integrationtime of the pulse pair correlators. This set of waveforms will besampled at a rate yielding at least two samples per chip period, andthen sent to a delay-hopped code detector.

[0037] The CDMA code correlator 85 in FIG. 8 will take samples of themultiple outputs of the bank of pulse pair correlators 83 ₁ to 83 _(n)and add them together in a manner dictated by the expected CDMA codeword. The objective of this operation is to produce the registered sumof all the chip signals. When the expected code word matches thetransmitted code word, this operation will have the effect of applying agating waveform, matched to the entire delay hopped (DH) code wordwaveform, to the observed data at the output of the correlators. If thegating waveform matches the shape of the chip signal waveform, a matchedfilter is implemented; however, this requires knowledge of the relativetiming of the sample clock and the transmitter chip clock. If the gatingwaveform applied to the individual chip is rectangular, with duration2T_(c), then the effect of the CDMA code word correlator is to add allof the individual chip waveforms in phase, producing an output which isa high-SNR version of the individual chip waveform.

[0038] The structure of one embodiment of the CDMA code correlator isdepicted in FIG. 11. The specific code correlator depicted uses a CDMAcode word that matches the correlator bank output depicted in FIG. 10.The code word correlator comprises multiple chip time delays(D_(chip time)) 111 ₁ to 111 _(N) _(c) , and a summer 112. Note that thechip time delays (D_(chip time)) and signs (additions and subtractions)cause the elementary correlator peaks to be aligned in time with thesame signs. The delayed outputs of the analog-to-digital converters(ADCs) from the CDMA code word correlator are summed by the summer andprovided as the output. Since the sample period of the ADCs has beenspecified to be a fraction of the chip period, the delays in FIG. 11may, in one embodiment, all be implemented as a number of digitalstorage devices, with provision for passing stored data from one to thenext. Thus, in one embodiment, the CDMA code word correlator of FIG. 11depicts a synchronous digital circuit such as would be implemented in aprogrammable logic device (PLD), such as a field programmable gate array(FPGA) or the like, or an application specific integrated circuit(ASIC).

[0039] Once the output samples of the code word correlator (representedby black diamonds in FIG. 12) have been formed, the receiver must decideif a code word has been received during the last sample interval. Ifthis decision is positive, other data must be derived from the samples.In the data transmission application of TR/DH, the code word would bemodulated by a ±1 which would represent the transmitted information.

[0040] For the application of this invention, the time at which the codeword was received is the most important piece of information. One way toestimate this value is to fit a model of the pulse-pair correlatoroutput waveform to the samples at the output of the code word generator.Such a fit could be done on the basis of minimum squared error, whichwould result in the optimum fit for Gaussian observation noise. Thepossible result of this algorithm is shown in FIG. 12, superimposed overthe sample values. The fitted model, which is triangular in shape tomatch the main lobe of the DH CDMA code correlator output function, iscontrolled by two parameters, the height of the peak, h, and thelocation in time of the peak, τ. This information can be supplemented bythe sum of squared errors for the best fit whose peak value is withinthe current sample interval. The absolute value of the peak value andthe sum of squared errors can be combined and compared to a threshold todetect the code word. The value of τ can be used as an estimate of thetime of arrival of the code word.

[0041] In particular, the minimum mean squared error estimate of theheight of the fitted triangle, given the DH code correlator output data{x₀, x₁, . . . , x_(N)} is given by$\hat{h\quad \varphi} = \frac{\sum\limits_{n - 0}^{N}{x_{n}{T\left( {n,\varphi} \right)}}}{\sum\limits_{n = 0}^{N}{T^{2}\left( {n,\varphi} \right)}}$

[0042] where the function T(n,φ) is a triangular model of the expectedwaveform. The first argument, n, is the sample number; the adjacentsamples of the model may be considered to be separated by the same timeinterval as are the data samples. There will be N+1 samples in themodel, corresponding to the number of samples expected in the mainlobeof the code correlator output waveform. The second argument of the modelis the relative phase of the model with respect to the samples used inthe multiplications above. The phase of the model can be explained byassuming that the model is sampled at some high rate, say M times theoutput sample rate of the code correlator, and so the entire model iscomposed of M(N+1) samples. M different sets of (N+1) model points canbe chosen, for which the model points are separated by M high-ratesamples. Each of these sets of model points can be regarded as adifferent phase of the model, for phases indexed φ=1, . . . , M.

[0043] When the receiver is looking for a TR/DH code word without anyprior synchronization information, the algorithm just described isexecuted for each new set of samples, that is, at the end of each sampleinterval. For each new sample, all phases of the model must be appliedto the last (N+1) saved data samples. When a set of results is computedfor which the height exceeds a pre-determined threshold and the modelingerror is lower than the error values computed for all near-by phases ofthe model, then we convert that sample number and phase into a time ofarrival for the TR/DH burst. The resulting time-of-arrival measurementis known relative to the ADC sample clock 87, which determines theoutput sample times of the DH CDMA code correlator 85.

[0044] It is worth noting that, for the impulse radio version of theinvention, the output of the pulse-pair correlator is only approximatelytriangular, even given an ideal finite-interval integrator. This isbecause the individual pulse-pair correlator output waveforms are notsmoothly triangular, but rather ascend and descend in discrete steps,rather than smoothly, as shown in FIG. 10. The locations of these stepsin time change randomly and correspond to the times of arrival ofindividual pulse pairs. It can be shown that the sum of such waveformsconverges to a triangle. On the other hand, for a noise carrier, thechip waveforms are triangular.

[0045] In the time-of-arrival (TOA) estimation method described above,what is actually measured is the time of the peak of the last chipsignal of the packet. This peak represents the time at which pulse-pairsseparated by a certain lag stop arriving, and that lag corresponds tothe lag of the last chip send to form the code word. If the transmittingdevice has only a direct-path transmission from the transmitter to thereceiver, then the time-of-arrival value will be determined by the timeof transmission and the distance between the receiver and thetransmitter involved.

[0046] On the other hand, any multipath will tend to spread out (intime) the peaks of the chip signals, which will have the effect ofdelaying the detected times of arrival relative to the direct path timesof arrival. This delay will amount to about half the observed multipathspread and is likely to be on the order of 10 to 50 ns for an indoorenvironment resembling an office building. (See Saunders, Antennas andPropagation for Wireless Communication Systems, John Wiley & Sons, 1999,pp. 282-285.)

[0047] Another potential source of inaccuracy in the TOA estimate isclock mismatch between the transmitter's chip clock and the receiver'ssample clock. Such a mismatch has the effect of shifting the locationsof the samples on the waveforms that emerge from the pulse-paircorrelators' integrators. Over the course of the reception of atransmitted TR/DH word, this precession of the phase of the sample clockwith respect to the phase of the received waveform has the effect ofsmearing out the output waveform in time. For example, if thetransmitted word is 400 microseconds long, and the transmit and receiveclock frequencies are mismatched by 10 PPM, then the composite waveformat the output of the CDMA code correlator will be smeared by 4nanoseconds. The expected value of the resulting TOA estimation errorwould be half that value. Unlike multipath, which produces onlyover-estimation errors, this precession in clock frequencies may resultin either over- or under-estimation of the TOA. Those skilled in the artwill appreciate that the maximum clock mismatch is determined by thestability of the oscillators used to produce the transmit and receiveclock waveforms. The maximum clock frequency mismatch and the allowableerror due to it will determine the maximum length of a word that may becoherently combined to form a TOA estimate, and therefore the maximumlength of a TR/DH preamble. The word length directly influences thedetection probability, and therefore the maximum transmission range.Such design trade-offs can be made by one skilled in the art.

[0048] In general, the accuracy of the time-of-arrival estimate willdecrease with the noise level and the multiple access interferencelevel. On the other hand, the accuracy will increase with the length ofthe code word, because the effective SNR of the final step will increasewith coding gain. The accuracy will also increase with the sample rate,because with more samples the error in fitting the model will decrease.

[0049] Experimentation with prototype TR/DH transmitters and receiversin an indoor environment has shown that the accuracy of the methoddescribed above is in the range of less than ten nanoseconds of error.In a typical indoor multipath situation, this means that the uncertaintyin the onset time of the inbound burst is still dominated by the two-waypropagation time.

[0050] Returning to FIG. 8, we will describe the mechanism for derivinga time mark from a TR/DH received word and using it to trigger atransmitter for the narrowband burst data transmission. The outputsamples of the DH CDMA code correlator 85 are input into a polyphasefilter and threshold logic module 86. This module implements the minimummean squared error computations given above. This is a polyphasecomputation in that, for every input sample, all the squared errorcomputation must be done for all the phases of the waveform model. Sincethe input to this module could be sampled at a rate as great as 20Msamples/second, this module will be implemented as a small ASIC or PLD.Alternatively, for lower sample rates and longer chip times, thisfunction could be implemented in a digital signal processor (DSP).

[0051] The output of the polyphase filter and threshold logic module 86would be conveniently expressed in the form of a sample number, relativeto the most recent sample, and a phase, which can be regarded as afraction of a sample period. This numerical data identifies a moment intime and must be converted into a trigger signal that starts thenarrowband transmitter at the proper time. In order for this operationto proceed, at least some of the data to be transmitted must have beengathered and buffered prior to generation of the start signal. Thefunction of generating the start signal is performed by the generatestart time signal block 88, which is most conveniently implemented by aDSP that has access to the sample clock 87 to which the numerical timemark is referenced. The start signal is input to the narrow bandtelemetry transmitter 89, which receives and buffers bits to betransmitted. The transmitter 89 is connected to its own antenna 90.

[0052] Note that, although the sample clock 87 is the only clock shownon the block diagram of FIG. 8, one or more higher frequency clocks willhave to be distributed to run the ASICs PLDs or DSPs used in theimplementation. These clocks are not shown in FIG. 8.

[0053] While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

What is claimed is:
 1. A method for synchronizing a radio telemetrysystem using radio frequency (RF) burst communications comprising thesteps of: generating at a base station a transmitted-reference (TR)pilot signal transmission; transmitting by the base station atransmitted-reference, delay-hopped (TR/DH) pilot signal to a pluralityof mobile devices in the radio telemetry system; using the TR/DH pilotsignal and a TR/DH receiver and time-of-arrival estimator in each of theplurality of mobile devices to acquire synchronization with the basestation; generating and buffering telemetry data at each of theplurality of mobile devices; and transmitting by each of the mobiledevices telemetry data at a predetermined time after synchronizationwith the base station.
 2. The method for synchronizing a radio telemetrysystem recited in claim 1, wherein the RF burst transmission is a burstof time modulated ultra-wideband (UWB) communications transmission. 3.The method for synchronizing a radio telemetry system recited in claim1, wherein the TR/DH pilot signal comprises a code word consisting ofN_(c) chips, transmitted sequentially, having a fixed duration, T_(c),each chip being composed of N_(p) pulse pairs.
 4. The method forsynchronizing a radio telemetry system recited in claim 1, wherein theTR/DH pilot signal comprises two noise signals transmitted for a timeperiod of duration T_(c).
 5. The method for synchronizing a radiotelemetry system recited in claim 1, wherein the transmissions from themobile devices to the base station are a narrowband modulation.
 6. Themethod for synchronizing a radio telemetry system recited in claim 5,wherein the narrowband modulation is Gaussian Minimum Shift Keying(GMSK).
 7. The method for synchronizing a radio telemetry system recitedin claim 1, wherein the telemetry system uses a time division, multipleaccess (TDMA) scheme for transmitting telemetry data.
 8. The method forsynchronizing a radio telemetry system recited in claim 1, wherein thetelemetry system uses a time slotted transmission scheme fortransmitting telemetry data.
 9. A radio telemetry system using radiofrequency (RF) burst communications comprising a base station and aplurality of mobile devices, apparatus for synchronizing the radiotelemetry system comprising: a pilot signal generator at the basestation for generating a transmitted-reference pilot signal, the basestation broadcasting a transmitted-reference, delay-hopped (TR/DH) pilotsignal to the mobile devices; a TR/DH receiver and time-of-arrivalestimator at each of the mobile devices for detecting the TR/DH pilotsignal to acquire synchronization with the transmitter; a data bufferfor temporarily storing telemetry data at each of the mobile devices;and a transmitter at each of the mobile devices for transmittingbuffered telemetry data at a predetermined time after synchronizationwith the base station.
 10. A radio telemetry system as recited in claim9, wherein the RF burst transmission is a burst of time modulatedultra-wideband (UWB) burst transmission.
 11. A radio telemetry system asrecited in claim 9, wherein the TR/DH pilot signal comprises a code wordconsisting of N_(c) chips, transmitted sequentially, having a fixedduration, T_(c), each chip being composed of N_(p) pulse pairs.
 12. Aradio telemetry system as recited in claim 9, wherein the TR/DH pilotsignal comprises two noise signals transmitted for a time period ofduration T_(c).
 13. A radio telemetry system as recited in claim 9,wherein the transmissions from the mobile devices to the base stationare a narrowband modulation.
 14. A radio telemetry system as recited inclaim 13, wherein the narrowband modulation is Gaussian Minimum ShiftKeying (GMSK).
 15. A radio telemetry system as recited in claim 9,wherein the telemetry system uses a time division, multiple access(TDMA) scheme for transmitting telemetry data.
 16. A radio telemetrysystem as recited in claim 9, wherein the telemetry system uses a timeslotted transmission scheme for transmitting telemetry data.
 17. A radiotelemetry system as recited in claim 9, wherein the TR/DH pilot signalcomprises a code word consisting of N_(c) chips, transmittedsequentially, having a fixed duration, T_(c), each chip being composedof N_(p) pulse pairs, the TR/DH receiver further comprising: a bank ofpulse pair correlators receiving the TR/DH pilot signal and generatingoutputs; a bank of analog-to-digital converters (ADCs) digitizing theoutputs of the bank of pulse pair correlators; a DH code word correlatorreceiving the digitized outputs from the bank of ADCs and generating acorrelation output; and time estimation logic receiving the correlationoutput of the DH code word correlator and generating timing informationfor generating a numerical time-of-arrival (TOA) estimate.
 18. Atelemetry system as recited in claim 17, wherein the DH code wordcorrelator is implemented as an application specific integrated circuit(ASIC).
 19. A telemetry system as recited in claim 17, wherein the DHcode word correlator is implemented as a programmable logic device(PLD).
 20. A telemetry system as recited in claim 17, wherein timeestimation logic detects the TR/DH pilot signal by estimating a TOA of aburst of RF composed of a TR/DH code word using a model.
 21. A radiotelemetry system as recited in claim 17, wherein the time estimationlogic is implemented using an application specific integrated circuit(ASIC).
 22. A radio telemetry system as recited in claim 17, wherein thetime estimation logic is implemented using programmable logic device(PLD).
 23. A radio telemetry system as recited in claim 17, wherein thetime estimation logic includes a programmable digital signal processor(DSP) to convert a numerical TOA estimate to a clock edge forcontrolling a start of transmission of telemetry data.
 24. A mobiledevice for a radio telemetry system including apparatus for initialsynchronization with a central transmitter comprising: a receiver fordetecting a transmitted-reference, delay-hopped (TR/DH) pilot signal toacquire synchronization with the central transmitter; a buffer fortemporarily storing telemetry data; and a transmitter for transmittingbuffered telemetry data at a predetermined time after synchronizationwith the central transmitter.
 25. A mobile device for a radio telemetrysystem as recited in claim 24, wherein the TR/DH pilot signal comprisesa code word consisting of N_(c) chips, transmitted sequentially, havinga fixed duration, T_(c), each chip being composed of N_(p) pulse pairs.26. A mobile device for a radio telemetry system as recited in claim 24,wherein the TR/DH pilot signal comprises two noise signals transmittedfor a time period of duration T_(c).
 27. A mobile device for a radiotelemetry system as recited in claim 24, wherein the transmissions fromthe mobile devices to the base station are a narrowband modulation. 28.A mobile device for a radio telemetry system as recited in claim 27,wherein the narrowband modulation is Gaussian Minimum Shift Keying(GMSK).
 29. A mobile device for a radio telemetry system as recited inclaim 24, wherein transmission of buffered telemetry data is based on atime division, multiple access (TDMA) scheme.
 30. A mobile device forradio telemetry system as recited in claim 24, wherein transmission ofbuffered telemetry data is based on a time slotted transmission txtvscheme.